Among various display devices, a liquid crystal display (LCD) device has been widely used in notebook computers, office automation apparatus, and audio/video apparatus because of their superior operational characteristics such as light weight and low power consumption. Specifically, an active matrix LCD (AM-LCD) device that employs switching elements and pixel electrodes arranged in a matrix structure is the subject of significant research and development because of its high resolution and superior suitability for displaying moving images.
For the purpose of displaying images in a liquid crystal panel of an LCD device, a common voltage is supplied to each pixel so that liquid crystal molecules rotate according to a voltage difference between a data signal supplied from a data driving unit and the common voltage.
FIG. 1 is a view showing a liquid crystal display device according to the related art. In FIG. 1, a liquid crystal display device includes a liquid crystal panel 60, a driving printed circuit board (PCB) unit 50, a plurality of gate driving units 82, 84 and 86 and a plurality of data driving units 72, 74 and 76. The driving PCB unit 50 includes a plurality of driving circuits such as a timing controller, a power supply and a gamma reference voltage generator and a common voltage compensation circuit unit 52 for supplying and compensating a common voltage. The common voltage compensation circuit 52 receives the common voltage passing through the liquid crystal panel 60 and compares the common voltage with a reference voltage or an initial common voltage. Further, the common voltage compensation circuit 52 generates a compensated common voltage on the basis of a difference between the common voltage and the reference voltage and supplies the compensated common voltage to the liquid crystal panel 60 again.
The liquid crystal panel 60 includes first and second substrates (not shown) and a liquid crystal layer (not shown) between the first and second substrates. A display area and a non-display area surrounding the display area are defined in liquid crystal panel 60 and the display area includes a plurality of pixel regions. A plurality of conductive dots, e.g., first to fourth conductive dots Ag1 to Ag4 including silver (Ag) are formed at edge portions of the non-display area of the first substrate. The common voltage supplied by the common voltage compensation circuit 52 is transmitted to a common electrode on the second substrate through the first to fourth conductive dots Ag1 to Ag4.
The plurality of data driving units may include first, second and third data driving units 72, 74 and 76 and the plurality of gate driving units may include first, second and third gate driving units 82, 84 and 86. Each of the first, second and third data driving units 72, 74 and 76 includes a data driving integrated circuit (IC) and is connected to the driving PCB unit 50 and one side of the liquid crystal panel 60. In addition, each of the first, second and third gate driving units 82, 84 and 86 includes a gate driving IC and is connected to the other side of the liquid crystal panel 60. For example, each of the first, second and third data driving units 72, 74 and 76 and the first, second and third gate driving units 82, 84 and 86 may include one of tape carrier package (TCP) and flexible printed circuit (FPC).
The common voltage outputted from the common voltage compensation circuit 52 is supplied to the liquid crystal panel 60 through the first data driving unit 72, and is transmitted to the first to fourth conductive dots Ag1 to Ag4 though first, second and third common lines VL1, VL2 and VL3. The first common line VL1 is formed in the non-display area corresponding to three sides of the liquid crystal panel 60. In addition, the second common line VL2 is formed in the first, second and third gate driving units 82, 84 and 86, and the third common line VL3 is formed in the non-display area corresponding to one side of the liquid crystal panel 60 adjacent to the first, second and third data driving units 72, 74 and 76. The common voltage transmitted to the first to fourth conductive dots Ag1 to Ag4 is applied to a common electrode of the second substrate. The common voltage as a feedback voltage is inputted to the common voltage compensation circuit 52 through the third data driving unit 76, and the common voltage compensation circuit 52 generates the compensated common voltage using the common voltage through the liquid crystal panel 60. The common voltage compensation circuit 52 supplies the compensated common voltage to the liquid crystal panel 60 again.
The first common line VL1 forms a first loop LOOP1 as a path for the common voltage. In addition, portions of the first common line VL1 and the second common line VL2 forms a second loop LOOP2 as a path for the common voltage, and the third common line VL3 between the first and fourth conductive dots Ag1 and Ag4 forms a third loop LOOP3 as a path for the common voltage. Accordingly, the common voltage is transmitted to the first to fourth conductive dots Ag1 to Ag4 through the first, second and third loops LOOP1, LOOP2 and LOOP3 of the first substrate and is applied to the common electrode of the second substrate.
However, since the first, second and third common lines VL1, VL2 and VL3 have a different length and a different arrangement, the first, second and third loops LOOP1, LOOP2 and LOOP3 have a different resistance and a different capacitance. For example, the common voltage through the third loop LOOP3 having a minimum length may have a minimum voltage drop. As a result, most of a current for the common voltage flows through the third loop LOOP3 and the compensated common voltage generated by the common voltage compensation circuit 52 reflects only the voltage drop in third loop LOOP3 and in a portion of the common electrode between the first and fourth conductive dots Ag1 and Ag4. Accordingly, the common voltage compensation circuit 52 compensates the voltage drop through an upper portion A of the liquid crystal panel 60 and does not compensate the voltage drop through middle and lower portions B and C of the liquid crystal panel 60. The voltage drop in the common voltage at the middle and lower portions B and C causes deterioration of display quality such as a greenish phenomenon.
In addition, the compensated common voltage is not completely applied to the entire portion of the common electrode due to the voltage drop. FIGS. 2A and 2B are views showing a compensated common voltage and a measured common voltage, respectively, of a liquid crystal display device according to the related art. In FIG. 2A, the compensated voltage generated by the common voltage compensation circuit 52 (of FIG. 1) changes from about −6V to about +6V according to a time. Although the compensated common voltage is supplied to the liquid crystal panel through the first data driving unit 72 (of FIG. 1), most of the current for the compensated common voltage flows through the third loop LOOP3 (of FIG. 1) from the first conductive dot Ag1 to the fourth conductive dot Ag4. As a result, the compensated common voltage is not properly applied to the middle and lower portions B and C of the common electrode due to the voltage drop in the first and second loops LOOP1 and LOOP2. Specifically, as shown in FIG. 2B, the measured common voltage at the lower portion C changes from about −2V to about +2V even when the compensated voltage of FIG. 2A is applied. As a result, the common voltage is not completely compensated at the entire portion of the common electrode, and deterioration such as a greenish phenomenon is caused.